Affects: Small intestine3–6 h
Lactose intolerance
The gut produces little lactase, the enzyme that splits milk sugar, and undigested lactose ferments in the colon.
What happens
Lactose needs the enzyme lactase to be split and absorbed. When lactase production falls, lactose reaches the colon intact, where bacteria ferment it and generate gas, water and acids. Hence the bloating, cramping and diarrhea, which appear between 30 minutes and two hours after consuming dairy.
It is the norm, not the exception
Most of humanity reduces lactase production after childhood; the ability to digest milk in adulthood is in fact a minority genetic adaptation. It is very common in Latin American and southern European populations.
It is not a milk allergy
They are different things. Intolerance is a digestive problem, dose-dependent and uncomfortable. Milk protein allergy is an immune reaction that can be serious. Confusing them leads to unnecessary restrictions or avoidable risks.
What helps
- Almost nobody needs to cut out dairy entirely: there is a personal tolerance threshold
- Aged cheeses and yogurt are usually better tolerated, as they contain less lactose
- Having dairy alongside other foods reduces symptoms
- Lactose-free products and lactase supplements exist
- Watch calcium and vitamin D intake if dairy is restricted
When something goes wrong
- Celiac diseaseAn autoimmune disease in which gluten triggers the immune system to attack the villi of the small intestine.
- SIBOAn excess of bacteria in the small intestine, which should host few. They ferment food before it is absorbed, producing gas and malabsorption.
- GERDAcidic stomach contents repeatedly rise into the esophagus because the sphincter between them does not close properly, irritating the lining.
